Method for sequentially transmitting a downlink signal from a communication station that has an antenna array to achieve an omnidirectional radiation

ABSTRACT

This invention relates to a method and apparatus for transmitting a downlink signal from a communication station to one or more subscriber units to achieve a desired radiation level over a desired sector (e.g., everywhere), the communication station including an array of antenna elements and one or more signal processors programmed (in the case of programmable signal processors) to weight the downlink signal according to one of a sequence of complex valued weight vectors. The method includes sequentially repeating transmitting the downlink signal, each repetition with a different weight vector from the sequence until all weight vectors in the sequence have been transmitted with. The sequence is designed for achieving the desired radiation level during at least one of the repetitions. In this way, every user in the desired region is transmitted to in the time period.

RELATIONSHIP TO OTHER PATENTS OR PATENT APPLICATIONS

This is a continuation in part to U.S. patent application Ser. No.08/988,519, filed on Dec. 10, 1997, entitled RADIO TRANSMISSION FROM ACOMMUNICATION STATION WITH AN ANTENNA ARRAY TO PROVIDE A DESIRABLERADIATION PATTERN (called the “Parent Patent” hereinunder). The ParentPatent is incorporated herein by reference in its entirety.

FIELD OF INVENTION

This invention relates to the field of wireless communication systems,and more specifically, to the efficient broadcast of common downlinkcommunication channel signals in a wireless communications system by acommunication station that uses a multiple element transmitting antennaarray in order to achieve a near omnidirectional pattern throughout itsarea of coverage.

BACKGROUND TO THE INVENTION

Cellular wireless communications systems are known, wherein ageographical area is divided into cells, and each cell includes a basestation (BS, BTS) for communicating with subscriber units (SUs) (alsocalled remote terminals, mobile units, mobile stations, subscriberstations, or remote users) within the cell. In such a system, there is aneed for broadcasting information from a base station to subscriberunits within the cell, for example to page a particular subscriber unitin order to initiate a call to that SU, or to send control informationto all subscriber units, for example on how to communicate with the basestation, the control information including, for example, base stationidentification, timing, and synchronization data. Such paging andcontrol information is broadcast on what are called common controlchannels. Because often there is no prior information regarding thelocation of the remote user(s) that need to receive the paging orcontrol information, or because such information is intended for severalusers, it is preferable to transmit such signals omnidirectionally, ornear omnidirectionally, where omnidirectional in general means that theradiated power pattern of the base station is independent of azimuth andelevation within the prescribed coverage area of the base station. Thisinvention deals with methods and apparatuses for achieving suchomnidirectional transmissions.

Some examples of a cellular system to which the present invention can beapplied are analog systems using the AMPS standard, digital systemswhich use variants of the Personal Handy Phone System (PHS) protocoldefined by the Association of Radio Industries and Businesses (ARIB)Preliminary Standard, RCR STD-28 (Version 2) December 1995, and digitalsystems that use the Global System for Mobile communications (GSM)protocol, including the original version, 1.8 GHz version calledDCS-1800, and the North American 1.9 GHz personal communications system(PCS) version called PCS-1900, these three called “variants” of GSMherein. The PHS and GSM standards define two general sets of functionalchannels (also called logical channels): a control channel (CCH) set anda traffic channel (TCH) set. The TCH set includes bi-directionalchannels for transmitting user data between the subscriber units and abase station. The CCH set includes a broadcast control channel (BCCH), apaging channel (PCH), and several other control channels not of concernherein. The BCCH is a unidirectional downlink channel for broadcastingcontrol information from the base station to the subscriber units thatincludes system and channel structure information, and the PCH is aone-way downlink channel that broadcasts information from the basestation to a selected set of subscriber units, or to a wide area ofmultiple subscriber units (the paging area), and typically is used toalert a particular remote station of an incoming call. The presentinvention is applicable to all downlink broadcasts and transmissions. Itis especially applicable for BCCH and PCH that are used by a basestation to simultaneously transmit common information to more than onesubscriber (i.e., to broadcast). It is also applicable to othersituations where it is desired to transmit RF energy omnidirectionallyor at least with no nulls anywhere in the intended region.

The use of antenna arrays for the radiation of radio frequency (RF)energy is well established in a variety of radio disciplines. For thepurposes of transmitting in the downlink from a base station whichincludes an antenna array to a remote receiver (the subscriber unit),the signal intended for the SU can be provided as input to each of theradiating elements of the array, differing from element to element onlyby gain and phase factors, usually resulting, by design, in adirectional radiation pattern focused at the subscriber unit. Thebenefits of this sort of transmission strategy include increased gainover that possible using a single radiating element and reducedinterference to other co-channel users in the system as compared totransmission by means of a single radiating element. Using such anantenna array, spatial division multiple access (SDMA) techniques alsoare possible in which the same “conventional channel” (i.e., the samefrequency channel in a frequency division multiple access (FDMA) system,timeslot in a time division multiple access (TDMA) system, code in acode division multiple access (CDMA) system, or timeslot and frequencyin a TDMA/FDMA system) may be assigned to more than one subscriber unit.

Any downlink signals sent are received by a subscriber unit, and thereceived signal at such receiving subscriber unit is processed as iswell known in the art.

When a signal is sent from a remote unit to a base station (i.e.,communication is in the uplink), the base station typically (and notnecessarily) is one that uses a receiving antenna array (usually, andnot necessarily the same antenna array as for transmission). The basestation signals received at each element of the receiving array are eachweighted in amplitude and phase by a receive weight (also called spatialdemultiplexing weight), this processing called spatial demultiplexing,all the receive weights determining a complex valued receive weightvector which is dependent on the receive spatial signature of the remoteuser transmitting to the base station. The receive spatial signaturecharacterizes how the base station array receives signals from aparticular subscriber unit in the absence of any interference. In thedownlink (communications from the base station unit to a subscriberunit), transmission is achieved by weighting the signal to betransmitted by each array element in amplitude and phase by a set ofrespective transmit weights (also called spatial multiplexing weights),all the transmit weights for a particular user determining a complexvalued transmit weight vector which also is dependent on what is calledthe “downlink spatial signature” or “transmit spatial signature” of theremote user which characterizes how the remote user receives signalsfrom the base station absence of any interference. When transmitting toseveral remote users on the same conventional channel, the sum ofweighted signals is transmitted by the antenna array. This invention isprimarily concerned with downlink communications, although thetechniques certainly are applicable also to uplink communications whenthe subscriber unit also uses an antenna array for transmitting andomnidirectional transmission from such a subscriber unit is desired.

In systems that use antenna arrays, the weighting of the signals eitherin the uplink from each antenna element in an array of antennas, or inthe downlink to each antenna element is called spatial processingherein. Spatial processing is useful even when no more than onesubscriber unit is assigned to any conventional channel. Thus, the termSDMA shall be used herein to include both the true spatial multiplexingcase of having more than one user per conventional channel, and the useof spatial processing with only one user per conventional channel. Theterm channel shall refer to a communications link between a base stationand a single remote user, so that the term SDMA covers both a singlechannel per conventional channel, and more than one channel perconventional channel. The multiple channels within a conventionalchannel are called spatial channels. For a description of SDMA systems,see for example, co-owned U.S. Pat. Nos. 5,515,378 (issued May 7, 1996)and 5,642,353 (issued Jun. 24, 1997) entitled SPATIAL DIVISION MULTIPLEACCESS WIRELESS COMMUNICATION SYSTEMS, Roy, III, et al., inventors, bothincorporated herein by reference; co-owned U.S. Pat. No. 5,592,490(issued Jan. 7, 1997) entitled SPECTRALLY EFFICIENT HIGH CAPACITYWIRELESS COMMUNICATION SYSTEMS, Barratt, et al., inventors, incorporatedherein by reference; co-owned U.S. patent application Ser. No.08/735,520 (filed Oct. 10, 1996), entitled SPECTRALLY EFFICIENT HIGHCAPACITY WIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING,Ottersten, et al., inventors, incorporated herein by reference; andco-owned U.S. patent application Ser. No. 08/729,390 (filed Oct. 11,1996) entitled METHOD AND APPARATUS FOR DECISION DIRECTED DEMODULATIONUSING ANTENNA ARRAYS AND SPATIAL PROCESSING, Barratt, et al., inventors,incorporated herein by reference. Systems that use antenna arrays toimprove the efficiency of communications and/or to provide SDMAsometimes are called smart antenna systems. The above patents and patentapplications are collectively referred to herein as “Our Smart AntennaPatents.”

Because broadcasting implies the simultaneous transmission of data overa common channel to a dispersed set of subscriber units, it is desirableto find methods for using the multiple element antenna array andassociated transmitter hardware for broadcasting both common downlinkchannel information and traffic information intended for one or moreparticular users.

Desirable Characteristics

A successful strategy will have the following characteristics:

given no prior information on the likely location of remote receivers, aremote receiver at any azimuth receiving a signal at least once over atime period at approximately the same level as a user at any otherlocation at any time during the time period. This is called “nearomnidirectional” (NOR) broadcasting herein;

low variation in the transmit power of each element in the array so thatgood advantage is taken of all elements in the array and scaling issuesthat arise in practice are minimized;

significant pattern gain relative to that achievable with a singleelement of the array transmitting at comparable power to the individualtransmission powers of the array elements in the time period; and

low total radiated energy so that all elements are being usedefficiently.

The property “low relative radiated power” herein means low radiatedpower per antenna element over a time period relative to the powerrequired to effect a comparable maximum radiation level (comparable inrange, azimuth and elevation) using a single antenna element of the samegain (e.g., as measured in dBi) as the individual elements of theantenna array. Since the difference in radiated power may translate todifferent power amplifier requirements, and very high power amplifiersare relatively expensive, in some situations, even 1 dB may be asignificant difference in radiated power. In more general cases, 3 dBwill be considered a significant difference in radiated power.

Sectorized systems using antenna arrays are known in the art. In asectorized system, rather than true omnidirectional broadcasting (360°of azimuth coverage) there is a need in the art for broadcastingefficiently in the intended coverage region (i.e., the sector) of theantenna array and associated electronics. Thus, in this document, theterm “omnidirectional” will be taken in the following sense: 1)“omnidirectional” means approximately, nearly omnidirectional (“NOR”);2) in an unsectorized cellular system, omnidirectional will mean NOR for360° of azimuth coverage, and 3) in a sectorized system, omnidirectionalwill mean nearly omnidirectional in the intended sector width (e.g.,120° of azimuth coverage for 120° sectors).

The Prior Art

A common method for so broadcasting data is to use an omnidirectionalantenna so that the RF carrier is transmitted more-or-less uniformly inall directions. This omnidirectional radiation pattern appears to be areasonable choice for mobile cellular systems in which the subscriberunits can be arbitrarily positioned within the cell area. In the case ofa smart antenna system, one can achieve such an omnidirectional patterneither by using a separate single omnidirectional antenna (such as avertical dipole) or one of the elements in the antenna array (assumed tohave m elements). Unfortunately, this would require increasing the totaltransmitter power in that antenna element (or separate antenna) comparedto the power levels used in ordinary TCH communications when all theantenna elements are operational, to achieve similar range for thetraffic and control channels. The option of increasing power may not beallowed by regulation and, even if allowed, may not be a practicalchoice because, for example, power amplifier costs tend to increaserapidly with power.

The prior art method of transmitting from only a single array elementwould satisfy the desirable criteria of approximately constant gain as afunction of azimuth and other quantities that describe the location ofthe remote receiver, and of low total radiated energy, but would notgive low variation in the transmit power of each element in the array sothat good advantage is taken of all elements in the array and scalingissues that arise in practice are minimized, and would not providesignificant pattern gain relative to that achievable with a singleelement of the array transmitting at comparable power to the individualtransmission powers of the array elements. In addition, transmittingfrom only one antenna would not enable simultaneous communications withseveral users on the same conventional channel.

Alternatively, the antenna array radiation pattern may be controlledthrough a combination of sending multiple beams and applyingpre-processing to any signals prior to beamforming. U.S. Pat. No.5,649,287, (issued Jul. 15, 1997), entitled ORTHOGONALIZING METHODS FORANTENNA PATTERN NULLFILLING, Forssen, et al., inventors, discloses amethod for sending information in a cellular communication systemcomprising at least one base station with an antenna array and aplurality of mobile stations. The common information is pre-processed tocreate orthogonal signals. The orthogonal signals are then beamformed sothat the orthogonal signals are delivered to the different beams in thearray antenna. The orthogonal signals are transmitted and then receivedat one or more mobile stations. The signals are then processed at themobile station to decipher the common information from the orthogonalsignals. The orthogonalizing signals to be transmitted to the mobilestations are formed so as to prevent nulls from occurring in the antennapattern.

The Forssen et al. method requires pre-processing (orthogonalizing) thecontrol signal to form m orthogonal signals which are then fed to abeamformer. That is, any signal to be broadcast is first transformed toa set of uncorrelated signals, and then each of these signals is sent ona different beam. This requires extra hardware or processing steps. Inaddition, the particular embodiment described by Forssen et al. requiresa high performance equalizer at the subscriber unit to resolve theorthogonalized signals from the other various lobes. It would bedesirable to use a system in which any signal to be transmitted isweighted only in phase and amplitude without requiring an additionalstep (e.g., orthogonalization).

Thus there is a need in the art for methods for omnidirectional downlinktransmitting that use the existing communications system apparatusincluding the existing antenna elements in an antenna array to achieveacceptable omnidirectional performance with low relative radiated power.There also is a need in the art for an apparatus that achieves this.

SUMMARY

One object of the invention is a method for downlink transmittingimplemented on a communication station that includes an array of antennaelements to achieve acceptable omnidirectional performance with lowrelative radiated power, omnidirectional in the sense that a remote userlocated anywhere in azimuth within the range of the communicationstation can receive the message over a period of time. Another object isan apparatus that achieves this.

These and other objects are provided for in the various aspects of thedisclosed invention.

One aspect of the invention disclosed herein is a method fortransmitting a downlink signal with a desirable radiation pattern tosubscriber units from a communication station which has an array ofantenna elements. In the communication station, there are one or moresignal processors programmed (in the case of programmable signalprocessors) to weight any downlink signal in phase and amplitude, theweighting describable as a complex valued weight vector. The weightedsignals are fed to the inputs of transmit apparatuses whose outputs arecoupled to the antenna elements. The method includes repeatingtransmitting the downlink signal a number of times, each transmissionincluding (a) applying a signal processing procedure from a set ofsignal processing procedures to form a processed downlink antennasignal, the processing procedure including weighting the downlink signalin phase and amplitude according to a weight vector, and (b)transmitting the downlink signals by passing each processed downlinkantenna signal to its intended antenna element through the intendedantenna element's associated transmit apparatus. The set of processingprocedures is designed so that any location in a desired sector achievesa desirable radiation level during at least one of the repetitions.Normally, the desired sector is a range of azimuths, for example, thewhole range in azimuths of the sector of the array, and the desiredradiation level is a non-null level. By a non-null level, we mean asignificant energy level so that reception is possible. That is, everyuser in any location is transmitted to in the time period for allrepetitions. Typically, the sequencing and each of the signal processingprocedures are carried out by running a program in one of the signalprocessors.

In one embodiment of the method, each of the set of signal processingprocedures comprises weighting with one of a sequence of differentweight vectors. The method includes carrying out for each weight vectorin the sequence of weight vectors the following steps: selecting a nextweight vector from the sequence, weighting the downlink signal in phaseand amplitude according to the selected weight vector to form a set ofweighted downlink antenna signals, and transmitting the downlink signalby passing each weighted downlink antenna signal to its intended antennaelement through the intended antenna element's associated transmitapparatus. The sequence is designed to achieve a desired radiation levelin any location over a desired sector during at least one of thesequential transmissions using the weight vectors sequence. Normally,the desired sector is the whole range in azimuth, and the desiredradiation level is a significant (i.e., non-null) level. That is, everyuser is transmitted to in the time period required to sequentialtransmit using all the weight vectors of the sequence. Typically, thesequencing logic is carried out by running a program in one of thesignal processors. In one implementation, the weights of the sequenceare pre-stored in a memory, and in another implementation, the weightsare computed on the fly, possibly from one or more prototype weights,which are stored in a memory.

In the particular embodiments disclosed, the communication stationoperates using the PHS air interface protocol in a cellular system. Onevariant of the system is for low mobility applications, while another isfor a wireless local loop (WLL) system. The invention, however, is notlimited to any particular multiplexing scheme or air interfacestandards. Other embodiments may use any analog or digital multiplexingscheme (e.g., FDMA, TDMA/FDMA, CDMA, etc.) and/or any air interfacestandards (e.g., AMPS, GSM, PHS, etc.).

In one embodiment disclosed, the elements of the sequence of weightvectors all have the same amplitude and have random phase. In oneimplementation, the random phase is achieved on the fly by randomizingmeans (e.g., a random phase generator) which may be included in thetransmit apparatuses. In another implementation, the sequence ispre-designed and pre-stored in a memory.

In another embodiment, the sequence is comprised of weight vectors thatare orthogonal. The orthogonal weight vectors preferably (and notnecessarily) have elements with the same magnitude. The descriptiondiscloses three examples of orthogonal sequences that may be used: asequence whose elements are the rows (or, equivalently, the columns) ofa complex valued Walsh-Hadamard matrix, a sequence whose elements arethe rows (or, equivalently, the columns) of a real valued Hadamardmatrix, and a sequence whose elements are the basis vectors of thediscrete Fourier transform (DFT or FFT).

In yet another embodiment, the sequence is comprised of weight vectorseach of which is designed to provide a desirable radiation pattern(e.g., a near omnidirectional (NOR) pattern) within a sub-sector of theoverall desired sector (typically the whole range in azimuth) with allthe sub-sectors covering the overall desired sector so that sequentiallybroadcasting with each weight in the sequence covers the whole desiredrange. The weight vectors of the sequence are designed using the methoddescribed in the Parent Patent (U.S. patent application Ser. No.08/988,519). In one embodiment, for example, the weight vectors of thesequence are each the weight vector that minimizes a cost function ofpossible weight vectors which includes an expression of the variationfrom the desirable radiation pattern of the radiation pattern within theparticular sub-sector resulting from transmitting using the weightvector. In a particular version applicable for the antenna array havingelements which are substantially uniformly distributed, a prototypeweight vector for one sub-sector is designed, and the other weightvectors of the sequence are “shifted” versions of the prototype obtainedby shifting the prototype weight vector by an amount determined by theangular shift of the sub-sector from the prototype weight vectorsub-sector. See the Parent Patent for details.

In another aspect of the invention, the sequence of weight vectorsincludes weight vectors that are representative of the weight vectorsdesigned for transmission to the known subscriber units for thecommunication station. Typically, the weight vectors designed fortransmission to the known subscriber units are determined from thetransmit spatial signatures of the known subscriber units. In oneembodiment, the representative weight vectors are the weight vectorsdesigned for transmission to the known subscriber units. In anotherembodiment, the representative weight vectors are fewer in number thanthe weight vectors designed for transmission to the known subscriberunits, and are determined from the subscriber units weight vector usinga vector quantization clustering method. Many clustering methods areknown in the art and any may be used for this part of the invention. Thepreferred embodiment clustering method starts with a set of weightvectors (e.g., the weight vectors designed for transmission to the knownsubscriber units) and iteratively determines a smaller set of weightvectors representative of the a set of weight vectors. At first, aninitial set of representative weight vectors is assigned. During eachiteration, each weight vector is combined with its nearestrepresentative weight vector, nearest according to some associationcriterion. An average measure of the distance between eachrepresentative weight vector and all the weight vectors combined withthat representative weight vector is determined. Preferably, the averagemeasure is the average square of the distance. Until the magnitude ofthe difference between this average measure in the present iteration andthis average distance in the previous iteration is less than somethreshold, each representative weight vector is replaced with a coreweight vector for all the weight vectors that have been combined withthe representative weight vector during that iteration, and thecombining and threshold checking step is repeated. The core weightvector preferably is the geometric centroid of all the weight vectorsthat have been combined with the representative weight vector duringthat iteration. When the average measure between each representativeweight vector and all the weight vectors combined with thatrepresentative weight vector is less than some threshold, therepresentative weight vectors achieving this are the finalrepresentative weight vectors used as the representative weight vectorsfor sequential transmission of the downlink signal.

In one embodiment of this clustering method, the association criterionused for nearness is the nearest Euclidean distance and the core weightvector is the geometric centroid of all the weight vectors that havebeen combined with the representative weight vector during thatiteration. In another embodiment, the association criterion used fornearness is the maximal cosine angle, in which case the core weightvector that each representative weight vector is replaced is theprincipal singular vector obtained from carrying out the singular valuedecomposition on all the weight vectors that have been combined with therepresentative weight vector during that iteration. In addition, in oneembodiment of the clustering method, the initial representative weightvectors are the unit amplitude weight vectors aimed at differentuniformly spaced angles in the angular region of interest (preferably360 degrees in azimuth). Other initial values also may be used. Forexample, in another embodiment applicable for the case of the number ofrepresentative weight vectors being equal to the number of antennaelements, one may use the Walsh-Hadamard orthogonal weights as theinitial set of representative weight vectors. Alternatively, one may useDFT orthogonal weights as the initial set of representative weightvectors.

In an alternate improved embodiment, the sequence of weight vectorsincludes two sub-sequences, the first sub-sequence comprising weightvectors which are representative of the transmit weight vectors for theexisting subscriber units, and the second sub-sequence including aweight vector designed for near omnidirectional broadcasting. The weightvector designed for near omnidirectional broadcasting may be so designedaccording an implementation of the method of the Parent Patent.Alternatively, the second sub-sequence may be a set of orthogonal weightvectors.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood from the detailedpreferred embodiments of the invention, which, however, should not betaken to limit the invention to any specific embodiment but are forexplanation and better understanding only. The embodiments in turn areexplained with the aid of the following figures:

FIG. 1 shows the transmit processing part and the transmit RF part of abase station on which the present invention may be embodied;

FIG. 2 shows the transmit processing part and the transmit RF part of abase station with post-processing means in the transmit path for eachantenna element;

FIGS. 3(a) and 3(b) show, in a simplified manner, the preferredembodiment clustering method at two different stages (iterations) of onemethod of selecting vector quantization code vectors from a set ofweight vectors;

FIG. 4 shows the results of carrying out a simulation without any weightsequencing. Three histograms of a PNLTY measure are shown in FIGS. 4(a),4(b) and 4(c) for γ (gamma) values of 0 (totally random), 0.5, and 1.0(totally geometric), respectively, each with a total of 10,000 trials;

FIGS. 5(a), 5(b) and 5(c) show simulation results for using DFT weightsequencing for the cases of γ=0, γ=0.5, and γ=1.0, respectively, eachwith 10,000 trials, according to one aspect of the invention; and

FIGS. 6(a), 6(b) and 6(c) show simulation results for using vectorquantized weight vector sequencing for the cases of γ=0, γ=0.5, andγ=1.0, respectively, each with 10,000 trials, according to anotheraspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention preferably is implemented in the base station part of awireless communication system with SDMA, in particular a cellular SDMAsystem. In one implementation, the system operates using the PHScommunications protocol which is suitable for low mobility applications.The subscriber units may be mobile. Above-mentioned andincorporated-herein-by-reference co-owned U.S. patent application Ser.No. 08/729,390 describes the hardware of a base station of such a systemin detail, the base station preferably having four antenna elements. Ina second implementation, the subscriber units have fixed location. ThePHS communications protocol again is used. Wireless systems with fixedlocations are sometimes called wireless local loop (WLL) systems. A WLLbase station into which some aspects of the present invention areincorporated is described in co-owned U.S. patent application Ser. No.09/020,049 (filed Feb. 6, 1998) entitled POWER CONTROL WITH SIGNALQUALITY ESTIMATION FOR SMART ANTENNA COMMUNICATION SYSTEMS, Yun,Inventor, incorporated-herein-by-reference (hereinafter “Our PowerControl Patent”). Such a WLL base station may have any number of antennaelements, and many of the simulations described herein will assume a12-antenna array. It will be clear to those or ordinary skill in the artthat the invention may be implemented in any SDMA system with one ormore than one spatial channel(s) per conventional channel, and havingmobile, fixed, or a combination of mobile and fixed subscriber units.Such a system may be analog or digital, and may use frequency divisionmultiple access (FDMA), code division multiple access (CDMA), or timedivision multiple access (TDMA) techniques, the latter usually incombination with FDMA (TDMA/FDMA).

FIG. 1 shows the transmit processing part and the transmit RF part of abase station (BS) on which the present invention may be embodied.Digital downlink signal 103 is to be broadcast by the base station, andtypically is generated in the base station. Signal 103 is processed by asignal processor 105 which processes downlink signal 103, the processingincluding spatial processing comprised of weighting downlink signal 103in phase and amplitude into a set of weighted downlink antenna signals,the weighting describable by a complex valued weight vector. Signalprocessor 105 may include a programmable processor in the form of one ormore digital signal processor devices (DSPs) or one or more generalpurpose microprocessors (MPUs) or both one or more MPUs and one or moreDSPs together with all the necessary memory and logic to operate. Thereader is referred to above-mentioned co-owned U.S. patent applicationSer. Nos. 08/729,390 and 09/020,049 for details. In the preferredembodiments, the spatial processing (spatial multiplexing) and themethods of the present invention are implemented in the form ofprogramming instructions in signal processor 105 that when loaded intomemory and executed in the DSP(s) or MPU(s) or both cause the apparatusof FIG. 1 to carry out the methods. Thus signal processor 105 has thesame number of outputs, that number denoted by m herein, as there areantenna elements in the transmitting antenna array of the base station.The outputs are shown as 106.1, 106.2, . . . , 106.m in FIG. 1. In thepreferred embodiment, the same antenna array is used for transmittingand for receiving with time domain duplexing (TDD) effected by atransmit/receive switch. Since the invention mainly is concerned withtransmitting, duplexing functionality is not shown in FIG. 1. FIG. 1thus would apply also for a base station that only transmits, for a basestation with different antennas for transmission and reception, and fora base station that uses frequency domain duplexing (FDD) with the sametransmit and receive antennas. The m outputs of signal processor 105,typically but not necessarily in baseband, are upconverted to therequired RF frequency, then RF amplified and fed to each of the mantenna elements 109.1, 109.2, . . . , 109.m. In the WLL and mobilesystems on which the invention is implemented, some of the upconversionis carried out digitally, and some in analog. Since upconversion and RFamplification is well known in the art, both are shown combined in FIG.1 as RF units 107.1, 107.2, . . . , 107.m.

GENERAL DESCRIPTION OF THE METHODS

The common aspect of the method and apparatus of the present inventionis to transmit the downlink signal a number of times, say n times, eachtime with different signal processing, the signal processing includingweighting with a transmit weight vector, and chosen so that over thetime to transmit with all the n different signal processing procedures,any location in a desired sector achieves a desirable radiation levelduring at least one of the transmissions. Normally, the desired sectoris a range of azimuths, for example, the whole range in azimuths of thesector of the array, and the desired radiation level is a significant(i.e., non-null level). Preferably, given no other information, a remoteuser in any azimuth in the desired sector sees the same maximumradiation level when the same distance from the transmitter over thetime to transmit with all the n different signal processing procedures.Usually the desired sector is 360° for a non-sectored system and thesector of the antenna array in a sectored system.

In one illustrative embodiment of the method of the present invention,the n instances of the signal processing each include weighting by acorresponding weight vector of a sequence of n transmit weight vectors.Thus, in this embodiment, the downlink signal is transmitted a number oftimes, say n times, each time with a different weight vector from asequence of n different weight vectors, the n weight vectors chosen sothat over the time to transmit with all the n weight vectors, anylocation in a desired sector (e.g., the sector of the array) achieves adesired radiation level during at least one of the transmissions.Normally, the desired sector is a range of azimuths, for example, thewhole range in azimuths of the sector of the array, and the desiredradiation level is a significant (ie., non-null) level. Preferably,given no other information, a remote user in any azimuth in the desiredsector sees the same maximum radiation level when the same distance fromthe transmitter over the time to transmit with all the n weight vectors.Usually the desired sector is 360° for a non-sectored system and thesector of the antenna array in a sectored system. While a differentweight vector effectively is used in each repetition, such a differencemay be achieved, for example, either by selecting a different weightvector, or using a single weight vector with additional means formodifying the weight vector to produce a different effective weightvector.

In another implementation, the signal processing procedure includespost-processing after the spatial processing, for example, using analogor digital filtering in baseband, or analog filtering in the RF domain,the spatial processing typically but not necessarily using essentiallythe same transmit weight vector for each repetition. In each of the ninstances of transmitting the downlink signal, the downlink signal isspatially processed to a plurality of signals, one for each antennaelement. Each of the antenna signals is post-processed in a differentway. Note that each of the antenna signals is upconverted to RF, usuallywith one or more stages of intermediate frequency (IF) amplification,and the processing may be done before such up-conversion, using digitalor analog means, or after digital upconversion (when there is digitalupconvension) using digital or analog means, or after analogupconversion using analog means. In the analog implementation, differentanalog filtering is introduced in each of the m antenna signals, and ineach of the n instances in RF units 107.1, 107.2, . . . , 107.m feedingthe m antenna elements 109.1, 109.2, . . . , 109.m. This may be done,for example, by introducing a different amount of time delay in each ofthe m antenna signals, and in each of the n instances. FIG. 2 showspost-processing means 203.1, 203.2, . . . , 203.m which, for example,are each time delay apparatuses which produce m different time delays.For each RF unit, the post-processing means is seen at the input.However, it would be clear to those in the art that the post-processingmight occur within the RF unit, and not only in baseband. When such timedelays are introduced, appropriate equalizers may be needed by receivingsubscriber units, as would be clear to those of ordinary skill in theart. The post processing may be done also, for example, by introducing adifferent amount of frequency offset in each of the m antenna signals,and in each of the n instances. FIG. 2 shows post-processing means203.1, 203.2, . . . , 203.m which in this case are each frequency offsetapparatuses which produce m different frequency offsets. The amounts ofdifferent frequency offset or different time delay to introduce in eachof the m antenna signals would be insufficient to cause problems for thedemodulators at the subscriber units but sufficient to orthogonalize them antenna signals. A particular frequency-offset introducing postprocessing embodiment may be used in systems that use programmableupconverter/filters in the RF transmit apparatuses. Such a device is theGraychip, Inc. (Palo Alto, Calif.) GC4114 quad digitalupconverter/filter device which is used in the implementation of RFsystems 107.1, 107.2, . . . , 107.m in the base station of the WLLsystem described in Our Power Control Patent, (above-mentioned U.S.patent application Ser. No. 09/020,049). The GC4114 has phase offset(and gain) registers which may be used to introduce frequency offsetinto the signal.

It should be mentioned that the frequency offset post-processing methodcan be thought of as transmitting with a transmit weight vector whosephase changes during the transmission time of each repetition. Forexample, with digital modulation such as used in the preferredembodiment, introducing a small frequency offset effectively causes theconstellation space to slowly rotate. The constellation space is thecomplex constellation swept out by a complex valued (in-phase componentI and quadrature component Q) baseband signal. Thus, using the frequencyoffset post-processing embodiment may cause different symbols of thedownlink signal burst to be transmitted with a different radiationpattern. Thus some averaging of the pattern occurs during eachrepetition, and it may be possible to use fewer repetitions.

Another way of introducing post-processing to produce a set oforthogonalized processed downlink signals to sequentially transmit is touse only one weight vector, and uses RF systems 107.1, 107.2, . . . ,107.m that each include means for randomizing the phase. The m phasesduring each transmission are then random with respect to each other.FIG. 2 shows post-processing means 203.1, 203.2, . . . , 203.m which inthis case are each phase randomizing means, included in RF systems107.1, 107.2, . . . , 107.m. For each RF unit, the phase randomizingmeans is seen at the input. However, it would be clear to those in theart that the randomizing might occur within the RF unit, and not only inbaseband. In one embodiment, randomizing means 203 includes sequentiallyaddressing sine and cosine lookup tables with random initial indexes.Another embodiment may be used in systems that use programmableupconverter/filters in the RF transmit apparatuses. For example, in theabove-mentioned embodiment which uses the Graychip, Inc. GC4114, whichhas phase offset (and gain) registers, these may be used to change thephase (and amplitude) of the signal. The phase change happens at thedigital IF.

A first illustrative apparatus embodying the invention includessequencing logic for sequencing through a sequence of n different weightvectors. In the preferred embodiment, the sequencing logic is a set ofprogramming instructions in signal processor 105 (which may consist ofone or more DSP devices). The sequencing means also includes, in oneembodiment, storage for storing the sequence of weight vectors, and inanother embodiment, generating means for generating the weight vectorsof the sequence of weight vectors on the fly, together with storagemeans for storing one or more prototype weight vectors from which thesequence is generated using the generating means. How to implement suchsequencing logic using DSP devices and/or microprocessors would be clearto one of ordinary skill in the art.

A second illustrative apparatus embodying the invention includessequencing logic for sequencing through a set of n signal processingprocedures. In the preferred embodiment, the sequencing logic and thesignal processing procedures are each a set of programming instructionsin signal processor 105 (which may consist of one ore more DSP devices).The signal processing procedures may be any of the sets ofpost-processing procedures described above for processing the spatiallyprocessed downlink signal into one of a set of orthogonal processeddownlink signals together with appropriate spatial processing. How toimplement such sequencing logic and signal processing using DSP devicesand/or microprocessors would be clear to one of ordinary skill in theart.

The PHS protocol used in the preferred embodiment allows one to definethe control channel interval (the amount of time between control bursts,in frames). For example, in many PHS systems, the control burst is sentevery 20 frames. Since a frame is 5 ms in standard PHS, this means theBCCH is sent every 100 ms. In PHS as used in the WLL systemincorporating the preferred embodiment, the control burst is sent every5 frames (25 ms). Therefore, if the sequence has 12 weights, then thecomplete sequence is repeated every 300 ms.

Random Phase Weights

In a first embodiment of using a sequence of weight vectors, the set ofweight vectors consists of weights of elements having the same amplitudewith randomly varying phase. Several ways are possible for implementingthis.

One way of achieving such random phase is to pre-choose and pre-store aset of weight vectors having the equal amplitude elements, but withrandom phases, and sequence through the set of weight vectors.

A second way of achieving random phase is to have one prototype weightvector, and to repeat transmission with the same weight vector modifiedon the fly to randomize the phase. Mathematically, denoting theprototype transmit weight vector by w with elements w₁, . . . , w_(m),the method includes repeating transmitting the downlink signal with aweight vector of elements w₁ exp(jφ₁), . . . , w_(m) exp(jφ_(m)), wherein each repetition, the φ₁, . . . , φ_(m) are varied randomly. That is,each of the quantities φ₁, . . . , φ_(m) is a random quantity uniformlydistributed between 0 and 2π.

Experiments were performed with the random phase strategy and it wasobserved that the statistics of the signal received by a stationary userapproximately followed a Raleigh distribution. A moving user receiving asignal from a base station transmitting with a single antenna would seesuch a distribution, for example. Therefore, standard communicationprotocols and air interface standards are particularly tolerant ofsignals that have Raleigh distributions.

Orthogonal Weights

A second embodiment uses a set of orthogonal weight vectors for thesequence of weight vectors. In the preferred embodiment, the number oforthogonal vectors to sequence through is equal to m, the number ofantenna elements in antenna array 109. Denote by w₁, i=1, . . . , m, theith (complex valued) transmit weight vector in the sequence. That is,for the duration of transmitting with the ith weight vector, themodulated signal to be broadcast is weighted (in baseband) in amplitudeand phase to each antenna element according to the value of thecorresponding complex valued element of weight vector w₁. Let s(t)denote the downlink signal to be broadcast, where t is time (either aninteger index for digital systems, or time in an analog system, as wouldbe understood by those of ordinary skill in the art). Let fn representthe necessary transmit modulation for the particular transmit system.For the PHS standard used in the preferred embodiments, fn isdifferential quartenary phase key modulation (DQPSK). Then, denoting

w _(i) =[w _(i1) . . . w _(im)],

the signal (e.g., in baseband), denoted y_(ij)(t), to be transmitted bythe jth antenna element (of a total of m antenna array elements) withthe ith weight may be mathematically described as

y _(ij)(t)=w _(ij) *fn(s(t)),

where ( )* indicates the complex conjugate.

A convenient way to specify all m weight vectors of the sequence is tostack up each of the w₁, i=1, . . . , m to form a m by m matrix denotedW, so that $W = {\begin{bmatrix}w_{1} \\w_{2} \\\vdots \\w_{m}\end{bmatrix}.}$

Specifying W specifies the whole sequence. W sometimes is referred to asthe basis matrix herein.

In the preferred embodiment, since it is desired to use all antennaelements, each (complex-valued) element of each weight vector in thesequence is forced to have the same magnitude. That is, all antennastransmit all the time (during the broadcast) with the same power.Mathematically, this can be expressed as |w_(ij)|=1 for all i and forall j. The actual magnitude is determined by the power control part ofthe base station. See for example, Our Power Control Patent(above-mentioned U.S. patent application Ser. No. 09/020,049).

Walsh-Hadamard Coefficients

In one embodiment, the weight vectors are the rows (or columns) of W,where W is a generalized (i.e., complex valued) Walsh-Hadamard matrix.The following MATLAB computer code (The Mathworks, Inc., Natick, Mass.),generates Walsh-Hadamard matrices for the cases of m=2, 4 and 8.

% % generating an orthogonal set of weights using a complex % version ofthe Walsh-Hadamard matrix. % the weight vectors can either be the row orcolumn vectors % of the basis matrix W. m = 4;   % m is the number ofantennas pos = [ 1+sqrt(−1) 1−sqrt(−1)]/sqrt(2); neg = [−1−sqrt(−1)1−sqrt(−1)]/sqrt(2); a2 = [pos; neg]; a4 = [a2 a2; a2 −a2]; a8 = [a4 a4;a4 −a4]; if (m == 2)  basis = a2; elseif (m == 4)  basis = a4; elseif (m== 8)  basis = a8; end;

In another embodiment, the weight vectors are the rows (or columns) ofm-dimensional matrix W, where W is a real-valued Hadamard matrix with +1and −1 coefficient values.

DFT Coefficients

In another embodiment, the weight vectors are the basis vectors of them-point discrete Fourier transform (DFT) and its fast implementation,the fast Fourier transform (FFT). These are the rows (or columns) of W,where, with j²=−1, $W = {\begin{bmatrix}1 & 1 & \cdots & 1 \\1 & ^{\frac{j2\pi}{m}} & \cdots & ^{\frac{{j2\pi}{({m - 1})}}{m}} \\\cdots & \cdots & \cdots & \cdots \\1 & ^{\frac{{j2\pi}{({m - 1})}}{m}} & \cdots & ^{\frac{{{j2\pi}{({m - 1})}}^{2}}{m}}\end{bmatrix}.}$

Methods Based on Weights with Desirable Patterns

The Parent Patent describes defining a cost function (of weightvector(s)) that defines the desirable aspects of the weight vectors interms of the overall radiation pattern, power distribution amongst theantenna elements, etc. Similarly, in another embodiment, a cost functionof a sequence of weight vectors is defined to achieve a desirableoverall pattern and a desirable variation in power amongst the variouselements of the transmitting antenna array.

One aspect of this is to split the design problem into a number ofdesigns of weight vectors, the weight vectors forming the sequence ofweight vectors. Each weight vector designed (for example using themethods described in the Parent Patent) for having a desirable radiationpattern over a sub-sector, the union of all the sub-sectors defining thedesired region of coverage and the superposition of all the sub-sectorsdefining the overall desired pattern over the region of coverage. Whenthe sequencing is in a particular order, this is equivalent to“sweeping” a region with a sub-sector, although there is no requirementto sequence in a particular order that simulates sweeping. When anapproximately uniform antenna array is used, a single “prototype” weightvector for achieving a near omnidirectional pattern over a single sector(say of width Δθ) is designed, and this weight vector is “shifted” by anamount defined by Δθ, the size of the overall desired region, and thenumber of weight vectors in the sequence. For example, with anapproximately uniform linear array for coverage over 180° with msequential transmissions, the shift is 180°/m and Δθ is preferablyslightly larger than 180°/m.

Another aspect is the more general design of a sequence of weightvectors that achieves the desired property directly, as defined by acost function to be minimized. How to design such a cost function wouldbe clear to one of ordinary skill in the art from this description andthat of the Parent Patent.

Methods Based on Knowledge of Remote Users

In a WLL system, subscriber unit locations are fixed and known (in theform of transmit spatial signatures) by the base station. One broadcaststrategy is based on sequentially transmitting the broadcast message toeach subscriber by using a weight vector determined from thesubscriber's known transmit spatial signature with possibly some othercriteria. Using a transmit weight vector determined only from thesubscriber's spatial signature may ensure that maximum power isdelivered to that user. An additional criterion to add may be minimizingenergy to other users.

For local subscriber loops with a large number of SUs, sequentiallytransmitting to all SUs may require too much time for each broadcastmessage. The amount of time required may be reduced by providing a setof sectorized radiation patterns (see above) that may be sequenced, inwhich each sectorized radiation pattern can cover more than onesubscriber. Another option is to determine a smaller set of broadcasttransmit weight vectors that adequately “represent” the set of weightvectors for each of the SUs. One example of this is vector quantization(VQ). See Gray, R. M., “Vector Quantization,” IEEE ASSP Magazine, Vol.1, No. 2, April 1984 (ISSN-0740-7467) for an introduction to VQ. VQmethods have been applied to other technical fields, such as to imagecompression, linear predictive coding of voice feature vectors forspeech coding and voice recognition, etc.

Let there be p remote users, the kth user having a transmit spatialsignature α_(tk), k=1, . . . , p. Let w_(k) be the weight vector “aimed”at the kth user. That is, if we express the spatial signatures in theform ${a_{tk} = \begin{bmatrix}{\alpha_{k1}^{{j\varphi}_{k1}}} \\{\alpha_{k2}^{{j\varphi}_{k2}}} \\\vdots \\{\alpha_{k\quad m}^{{j\varphi}_{k\quad m}}}\end{bmatrix}},$

where the α_(ki) are positive amplitudes, and the φ_(ki) are angles,then the “optimal” weight vectors “aimed” at the p users are${w_{k} = \begin{bmatrix}\begin{matrix}\begin{matrix}^{- {j\varphi}_{k1}} \\^{- {j\varphi}_{k2}}\end{matrix} \\\vdots\end{matrix} \\^{- {j\varphi}_{k\quad m}}\end{bmatrix}},{k = 1},\ldots \quad,{p.}$

Many methods are known for selecting the set of n representativem-vectors of a larger set of p m-vectors. Amongst these are whatgenerally are known as “clustering” methods in the literature. In ourapplication, one starts with p weight vectors (for example, the p weightvectors aimed at the p known remote users), and determines from thesethe n weight vectors (code vectors) that are representative of the pweight vectors. The particular clustering method used is now described.Note that while p is preferably the number of remote users, the methodis general; there may be more initial weight vectors than known remoteusers (see later). The method we use proceeds iteratively as follows:

1. Start with p weight vectors (these are denoted w₁, i=1, . . . , p),and preferably p is the number of remote users and the w_(i) are theoptimal weight vectors aimed at the p remote users, and start with the ninitial code vectors (denoted v_(k), k=1, . . . , n). Preferably, theinitial code vectors are the unit amplitude weight vectors aimed at nuniformly spaced angles in the angular region of interest (preferably360 degrees in azimuth).

2. For each of the weight vectors, (i.e., for each i=1, . . . , p), findk such that ∥w_(i)−v_(k)∥≦∥w_(i)−v_(l)∥ for all l=1, . . . , n. Thisfinds for each weight vector w₁, the nearest neighbor code vector v_(k)(“nearest” in Euclidean distance ∥ . . . ∥). The criterion used here iscalled the “association” criterion, so the association criterionpreferably is the nearest Euclidean distance.

3. Combine (associate) each such weight vector w_(i) with its thenearest neighbor code vector v_(k). Denote the number of weight vectorsthat are combined with the code vector v_(k) by n_(k) and denote asw_(i,k) the weight vectors w_(i) that have been combined with codevector v_(k).

4. Calculate the average squared Euclidean distance between the weightvectors and the code vectors they are combined with. That is, calculate$d^{2} = {\frac{1}{p}{\sum\limits_{k = 1}^{n}{\sum\limits_{i = 1}^{n_{k}}{{w_{i,k} - v_{k}}}^{2}}}}$

and determine if the magnitude of the difference between d² for thisiteration and the value of d² for the previous iteration is less thansome small threshold δd². If yes, stop. In one embodiment, δd² is 10¹²when all weight vectors are normalized to 1. Note that step 4 need notbe carried out in the first iteration.

5. If this is the first iteration or if the magnitude of the differencein d² between the current and the previous iteration is not less thanthe threshold δd², replace each code vector v_(k), k=1, . . . , n, bythe geometric centroid (in m-dimensional complex space) of the n_(k)weight vectors w_(i,k) that have been combined with that code vectorv_(k). That is, replace each v_(k) with$v_{k,{new}} = {\frac{1}{n_{k}}{\sum\limits_{i = 1}^{n_{k}}{w_{i,k}.}}}$

6. Go back to step 2.

Thus one determines n vectors representative of the p weight vectors,these p vectors preferably being the weight vectors optimal for theknown remote users.

In an alternate implementation, the association criterion in determiningneighbors step 2 and in combining step 3 is to combine each weightvector w_(i) with its maximal cosine angle code vector v_(k) rather thanwith its nearest neighbor code vector. The cosine of the angle betweentwo vectors is the dot product of the normalized vectors:${{\cos \quad \theta_{i,k}} = \frac{{w_{i} \cdot v_{k}}}{{w_{i}} \cdot {v_{k}}}},$

where • is the dot product. Step 5 of replacing each code vector in thiscase is modified to: carrying out a singular value decomposition (SVD)on the matrix whose column are the n_(k) weight vectors w_(i,k) thathave been combined with code vector v_(k), and replacing each codevector v_(k), k=1, . . . , n, by the principal singular vector obtainedfrom carrying out the SVD on the n_(k) weight vectors w_(i,k) that havebeen combined with code vector v_(k).

Other initial values also may be used. For example, in yet anotheralternate implementation, applicable for the case of the number of codevectors n being equal to the number of antenna elements m, one may usethe Walsh-Hadamard orthogonal weights as the initial set of codevectors. Alternatively, one may use DFT orthogonal weights as theinitial set of code vectors. Alternatively, one may use a set of randominitial code vectors.

An example of using the method of selecting the code vectors using thepreferred method is shown in FIGS. 3(a) and 3(b) in two dimensions forvisual simplicity. In practice, of course, the vectors are complexvalues and in m dimensions. In the case shown, there are twelve originalweight vectors and from these, four (the number n) code vectors aregenerated. FIGS. 3(a) and 3(b) show the state of the method at twodifferent stages (iterations) of the preferred code vector generationmethod. The four code vectors are shown as circles, and are numbered333, 335, 337, and 339 in FIG. 3(a) and 343, 345, 347, and 349 in FIG.3(b), respectively. The twelve original weights are shown as Xs in bothfigures. Some initial set of code vectors is assigned initially, andthese are code vectors 333, 335, 337, and 339 of FIG. 3(a). During eachiteration, each weight vector is combined with its nearest code vector,splitting space into four regions. The boundaries of the regions areshown as dotted lines 303 in FIG. 3(a) and dotted lines 313 in FIG.3(b), and the regions are shown labeled P1 ₁-P4 ₁ and P1 ₂-P4 ₂ in FIGS.3(a) and 3(b) respectively. The code vectors in any stage, e.g., codevectors 343, 345, 347, and 349 in FIG. 3(b), are the centroids of theweight vectors of each former region. Thus, code vector 345 in FIG. 3(b)is the centroid of the four weight vectors in region P2 ₁ in FIG. 3(a).Replacing the code vectors by the centroids, the average Euclideandistance between code vectors and original weight vectors woulddecrease. The preferred embodiment method stops when the differencebetween the average Euclidean distance at the present stage (iteration)i and the previous iteration is smaller than some predefined threshold.The n weight vectors used for sequencing are the code vectors of thelast iteration.

An alternative method for determining the n code vectors to use as thesequence of weight vectors with which to sequentially transmit thedownlink methods from an initial set of p weight vectors is based onusing the singular value decomposition (SVD). The SVD method applied tothe code vector selection process proceeds recursively as follows:

1. Perform the singular value decomposition on the matrix [w₁ . . .w_(p)] whose columns are the p weight vectors. As before, preferably pis the number of remote users and the w_(i) are the optimal weightvectors aimed at the p remote users. Consider the principal singularvector, denoted by x.

2. For each of the p weight vectors w₁, . . . w_(p), determine thecosine of the angle between the weight vector and the principal singularvector, that is, determine${{\cos \quad \theta_{i,x}} = \frac{{w_{i} \cdot x}}{{w_{i}} \cdot {x}}},{{{for}\quad i} = 1},\ldots \quad,{p.}$

3. Split the set of weight vectors into two sets. If the cosine of theangle between a weight vector and the principal singular vector issmaller than some threshold, that weight vector is selected for thefirst set. Otherwise, that weight vector is assigned to the second set.

4. Repeat the above steps 1, 2 and 3 for the second set to split it upinto two sets, continuing this recursion step 4 until the number of setsn is obtained, and the code vectors are then the n principal singularvectors from the recursions.

Other methods for determining the n code vectors to use as the sequenceof weight vectors with which to sequentially transmit the downlinkmethods also may be used without deviating from the scope. See forexample the above-mentioned article by R. M. Gray. See also, forexample, the binary split method of Rabiner, L. R., et al., “Note on theproperties of a Vector Quantizer for LPC Coefficients”, Bell SystemsTechnical Journal, Vol. 62, No. 8, October 1983, pp. 2603-2616. This andother methods of “clustering” known in the art may be adapted to thecommon channel broadcast problem, and how to so adapt a clusteringmethod would be clear to those of ordinary skill in the art.

While the above discussion assumes p remote users and p initial weightvectors, there also may be more than one weight vector per remote user,so that p may be larger than the number of known remote users. Forexample, in a typical system, some of the remote users' spatialsignatures may change significantly over time, while others do not.Thus, in an alternate embodiment of the VQ method (applicable to allalternative VQ implementations), the original weight vectors from whichto determine the representative set of n weight vectors include a recordover time of the weight vectors of users. In another alternateembodiment, a statistical record of remote user weight vectors is used.

To implement this in the WLL system for which some of the alternateembodiments of the invention are candidates, typically 6 or 7 spatialsignatures may be stored for each remote user. In addition, theshort-term (over one call) and long-term (over several calls) varianceof the spatial signatures may be stored.

In these embodiments, the generation of the n code vectors to use forthe sequential broadcasting method of the invention is carried outperiodically as the user base is known to change. This generation may becarried out off-line, or may be carried out within the base station insignal processor 105.

Alternatively, p may be less than the number of known remote users. Forexample, one of the p weight vectors may be sufficient to cover morethan one remote user.

Methods Based on Partial Knowledge

While in general in a WLL system, the spatial signatures of the existingremote users are known, there may be some new users in the system whosesignatures are not yet known. In an improved embodiment, the message issequentially transmitted with each weight vector of a first set of nweight vectors which are representative of the existing remote users,and then the message is broadcast again with an additional weight vectorfrom a second set of some other number, say n₁, of weight vectorsdesigned for (near) omnidirectional broadcasting, e.g., weight vectorswhich are either orthogonal, or are randomized (e.g., random phase) asdescribed herein above. Sequential transmissions with the nrepresentative weight vectors are now repeated before transmission withthe next weight of the second set of weight vectors. In this way, thedownlink message will eventually be received by even an unknown remoteuser, this typically taking longer than to be received by a known remoteuser.

In an alternate improved embodiment, the message is sequentiallytransmitted with each weight vector of a first set of n weight vectorswhich are representative of the existing remote users, and then themessage is broadcast with an additional weight vector designed for nearomnidirectional broadcast, for example using any of the embodimentsdescribed in the Parent Patent.

In the case of a cellular system serving mobile subscriber units, it isnot possible to assign fixed transmit weight vectors because locationvaries with time. However, a set of preferred locations may developbecause of subscriber “attractor” locations, such as airports or othertransportation centers, that tend to be temporary locations for asignificant fraction of the subscriber mobile stations within thecoverage area at any give time.

If a particular base station serves both stationary and mobilesubscribers, a combination strategy can be used to serve both types ofsubscribers by sequencing through a codebook set of VQ weight vectorsrepresentative of the weight vectors for the users with known spatialsignatures, and then by sequencing through an appropriate set of randomphase or of orthogonal weight vectors designed for near omnidirectionalbroadcasting. Also, attractor locations that tend to have a large numberof mobile clients, such as transportation centers, having knownassociated transmit weight vectors can be included together with thestationary subscriber units in the VQ process, or accessed in additionto other antenna radiation pattern sequencing. Sectors with differentconcentrations of subscriber units can be treated differently, e.g.,generating multiple VQ codebooks for broadcasting to different sectorsseparately or in combination with other sequencing strategies.

Simulation Results

Some of the methods described hereunder were evaluated by simulation. Inthe simulation, the “optimal” weight vectors to use are known, where“optimal” is defined below. The transmit spatial signature characterizeshow a remote terminal receives signals from each of the antenna arrayelements at the base station over a particular conventional channel. Inone embodiment, it is a complex valued column vector, denoted α_(t)herein, containing relative amounts (amplitude and phase with respect tosome fixed reference) of each of the antenna element transmitter outputsthat are contained in the output of the receiver at the remote terminal.For an m-element array,

α_(t)=[α_(t1)α_(t2) . . . α_(tm)]^(T),

where ( ) ^(T) is the transpose operation, and α_(tj), j=1, . . . , m,are the amplitude and phase (with respect to some fixed reference) ofthe remote terminal receiver output for a unit power signal transmittedfrom the jth antenna element of the base station to the remote terminal.Thus, in the absence of any interference and noise, when a signaly_(ij)(t)=w_(ij)*fn(s(t)) is sent by the jth antenna element (of a totalof m antenna array elements) with the ith transmit weight of thesequence of weights, then the signal z_(i)(t) at the remote terminalreceiver output is${z_{i}(t)} = {{{{fn}\left( {s(t)} \right)}{\sum\limits_{j = 1}^{m}{w_{ij}^{*}a_{tj}}}} = {{{fn}\left( {s(t)} \right)}w_{i}^{*}{a_{t}.}}}$

To optimally send a signal to this remote user with transmit spatialsignature α_(t), one chooses a weight vector w that maximizes thereceived power at the remote terminal, i.e., the w that maximizes|w*α_(t)|² or |w*α_(t)|, subject, for example, to a constraint to thetotal radiated power. This is what is called the “optimal” weight vectorin the above paragraph. Denoting such a weight vector as w_(opt), onecriterion to use for assessing the effectiveness of the weight vectorsequence is to calculate for all of the remote users (each having aparticular spatial signature α_(t)), a penalty figure PNLTY defined as${PNLTY} = {20{\log_{10}\left( \frac{{w_{opt}^{*}a_{t}}}{\max\limits_{i}{{w_{i}^{*}a_{t}}}} \right)}\quad {in}\quad {{dB}.}}$

A lower value of PNLTY is desired.

In the simulations to test some of the aspects of the invention, eachspatial signature (associated with a remote user) is assumed to be madeup of a “geometric” part and a “random” part. The geometric part takesinto account the relative phase delays between the waves that aretransmitted from each element in the antenna array towards the remoteuser. The remote user is assumed to be in the far field of each of theantenna elements. The geometric transmission medium is assumed isotropicand non-dispersive so that the radiation travels in straight lines tothe remote user, and the remote user is assumed to be far away from thebase station so that the direction of the remote user from each of theantenna elements is the same angle. In addition, the transmitted signalsare assumed to be narrowband and have all the same carrier frequency.

The random part of any spatial signature is made up of real andimaginary parts, these each being Gaussian distributed random variablesof 0 mean and some variance. In the simulations, any (complex valued)transmit spatial signature thus is assumed to take on a form

α_(t)=γα_(tG)+(1−γ)α_(tR)

where α_(tG) is the geometric part, α_(tR) is the random part, and γ isa parameter herein called the “clutter rating” and takes on a value ofbetween 0 and 1. Thus, a value of γ=0 means a totally random spatialsignature, while a value of γ=1 means a totally geometric spatialsignature for the simulations used to test the various embodiments ofthe invention.

FIG. 4 shows the results of carrying out a simulation without any weightsequencing. The antenna array for the simulations consists of twelveelements spaced uniformly around a circle. Three histograms of the valueof PNLTY are shown, each with a total number of N spatial signaturevalues, where N=10,000, are shown in FIGS. 4(a), 4(b) and 4(c) for γ(gamma) values of 0 (totally random), 0.5, and 1.0 (totally geometric),respectively. The horizontal axis is the Penalty measure PNLTY. With noweight sequencing, the mean value of PNLTY is 14.6 dB, 15.0 dB and 29.1dB for the cases of γ=0, γ=0.5, and γ=1.0, respectively. In addition, amargin of between 16.0 dB to 19.8 dB, depending on how the channelspatial signature is simulated, is necessary to reach 80% of thesubscriber units simulated.

The simulations results when DFT weight sequencing is used can be seenon FIGS. 5(a), 5(b) and 5(c) for the cases of γ=0, γ=0.5, and γ=1.0,respectively with 10,000 trials. With DFT weight sequencing, the meanvalue of PNLTY is 5.1 dB, 5.2 dB and 7.3 dB for the cases of γ=0, γ=0.5,and γ=1.0, respectively. In addition, the margins necessary to reach 80%of the subscriber units simulated are 6.1 dB to 8.8 dB, depending ongamma. This is a significant improvement from the case of no sequencing.

The simulations were carried out also using the vector quantizationmethod of the preferred embodiment, with the number of code vectors nequal to the number of antenna elements m. That is, n=m=12. Theuniformly distributed directions weight vectors were used as the initialset of code vectors, and the Euclidean distance (norm) was used as theassociation criterion. The simulations results can be seen in FIGS.6(a), 6(b) and 6(c) for the cases of γ=0, γ=0.5, and γ=1.0,respectively, again with 10,000 trials. With such code vectorsequencing, the mean value of PNLTY is 5.4 dB, 5.0 dB and 4.0 dB for thecases of γ=0, γ=0.5, and γ=1.0, respectively. In addition, the marginsnecessary to reach 80% of the subscriber units simulated are 5.3 dB to6.4 dB, depending on gamma. Again, this is a significant improvementfrom the case of no sequencing.

As will be understood by those skilled in the art, the skilledpractitioner may make many changes in the methods and apparatuses asdescribed above without departing from the spirit and scope of theinvention. For example, the communication station in which the method isimplemented may use one of many protocols. In addition, severalarchitectures of these stations are possible. Many more variations arepossible. The true spirit and scope of the invention should be limitedonly as set forth in the claims that follow.

What is claimed is:
 1. A method for transmitting a downlink signal froma communication station to one or more subscriber units, thecommunication station including an array of antenna elements, eachantenna element coupled to an associated transmit apparatus having aninput and an output, the coupling of each antenna element being to theoutput of its associated transmit apparatus, the associated transmitapparatus inputs coupled to a signal processor, the method comprising:for each particular signal processing procedure of a set of differentsignal processing procedures, each of the signal processing proceduresbeing for processing the downlink signal to form a plurality ofprocessed downlink antenna signals, each of the signal processingprocedures including weighting the downlink signal in phase andamplitude according to a corresponding weight vector, each processeddownlink antenna signal having an intended antenna element in the array,repeating the steps of: (a) processing the downlink signal according tothe particular signal processing procedure to form a particularplurality of processed downlink antenna signals; (b) transmitting thedownlink signal by passing each processed downlink antenna signal of theparticular plurality of processed downlink antenna signals to itsintended antenna element through the intended antenna element'sassociated transmit apparatus the set of different signal processingprocedures designed to achieve a desirable radiation level at anylocation in the complete range of azimuths of the antenna array duringat least one of the repetitions of step (b) of transmitting, eachcorresponding weight vector used for weighting in each differentprocessing procedure being a different weight vector of a sequence ofdifferent weight vectors, the weighting according to each correspondingweight vector producing the plurality of processed downlink antennasignals, the sequence of weight vectors designed to achieve a desirableradiation level at any location in a desired sector during at least oneof the repetitions of step (b) of transmitting.
 2. The method accordingto claim 1 wherein the weight vectors of the sequence of weight vectorsare pre-stored in a memory.
 3. The method according to claim 1, whereinthe weight vectors of the sequence of weight vectors are computed from aset of one or more prototype weight vectors, the set of prototype weightvectors being pre-stored in a memory.
 4. The method according to claim 1wherein the weight vectors of the sequence of weight vectors each haveelements that have the same amplitude and have random phase.
 5. Themethod according to claim 1 wherein the elements of each of the weightvectors of the sequence of weight vectors have equal magnitude.
 6. Themethod according to claim 1 wherein the number of weight vectors in thesequence of weight vectors is the same as the number of antennas, thenumber of antennas denoted by m, and the weight vectors of the sequenceof weight vectors are orthogonal.
 7. The method according to claim 6wherein the elements of each of the weight vectors of the sequence ofweight vectors have equal magnitude.
 8. The method according to claim 6wherein the weight vectors of the sequence of weight vectors are formedfrom the rows of a complex valued m-dimensional Walsh-Hadamard matrix.9. The method according to claim 6 wherein the weight vectors of thesequence of weight vectors are formed from the rows of a real valuedm-dimensional Hadamard matrix.
 10. The method according to claim 6wherein the weight vectors of the sequence of weight vectors are formedfrom the basis vectors of a m-dimensional discrete Fourier transform(DFT).
 11. The method according to claim 6 wherein each of the weightvectors of the sequence of different weight vectors is designed toprovide a particular desirable radiation pattern within a sub-sector ofthe overall desired sector, all the sub-sectors covering the overalldesired sector, each weight vectors minimizing a cost function ofpossible weight vectors which includes an expression of the variationfrom the particular desirable radiation pattern of the radiation patternwithin the particular sub-sector resulting from transmitting using theweight vector.
 12. The method according to claim 11 wherein the antennaarray has elements which are substantially uniformly distributed, aprototype weight vector for one sub-sector is designed, and the otherweight vectors of the sequence are shifted versions of the prototypeobtained by shifting the prototype weight vector by an amount determinedby the angular shift of the sub-sector from the prototype weight vectorsub-sector.
 13. The method according to claim 11 wherein the antennaarray has elements which are substantially uniformly distributed, aprototype weight vector for one sub-sector is designed, and the otherweight vectors of the sequence are shifted versions of the prototypeobtained by shifting the prototype weight vector by an amount determinedby the angular shift of the sub-sector from the prototype weight vectorsub-sector.
 14. The method according to claim 13 wherein the set ofrepresentative weight vectors included in the sequence has fewer weightvectors than the number of known subscriber units.
 15. The methodaccording to claim 14 wherein the representative weight vectors aredetermined from the weight vectors designed for transmission to theknown subscriber units, the determining of the representative weightvectors from the designed-for-subscriber-unit weight vectors using avector quantization clustering method.
 16. The method of claim 15wherein the clustering method includes: (i) assigning an initial set ofweight vectors as a current set of representative weight vectors; (ii)combining each designed-for-subscriber-unit weight vector with itsnearest representative weight vector in the current set, nearestaccording to some association criterion; (iii) determining an averagemeasure of the distance between each representative weight vector in thecurrent set and all the weight vectors combined with that representativevector; (iv) replacing each representative weight vector in the currentset with a core weight vector for all the weight vectors that have beencombined with that representative weight vector; (v) iterativelyrepeating steps (ii), (iii) and (iv) until the magnitude of thedifference between the average measure in the present iteration and theaverage distance in the previous iteration is less than a threshold, theset of representative weight vectors being the current set when themagnitude of the difference is less than the threshold.
 17. The methodof claim 16 wherein the association criterion for nearness is theEuclidean distance and the core weight vector is the geometric centroidof all the weight vectors that have been combined with therepresentative weight vector of the current set of representative weightvectors during that iteration.
 18. The method of claim 16 wherein theaverage measure is the average square of the distance.
 19. The method ofclaim 16 wherein the association criterion used for nearness is themaximal cosine angle and the core weight vector is the principalsingular vector obtained from carrying out the singular valuedecomposition on all the weight vectors that have been combined with therepresentative weight vector of the present set of representative weightvectors during that iteration.
 20. The method of claim 16 wherein theinitial set of weight vectors are the unit amplitude weight vectorsaimed at different uniformly spaced angles in the desired sector. 21.The method of claim 13 wherein the set of representative weight vectorsforms a first sub-sequence of the sequence of weight vectors and thesequence of weight vectors further comprises a second sub-sequence ofweight vectors.
 22. The method of claim 13 wherein the secondsub-sequence comprises a particular weight vector designed to provide aparticular desirable radiation pattern in the desired sector, theparticular weight vectors minimizing a cost function of possible weightvectors which includes an expression of the variation from theparticular desirable radiation pattern of the radiation pattern withinthe sector resulting from transmitting using the weight vector.
 23. Themethod of claim 22 wherein the particular desirable radiation pattern isa near omnidirectional pattern.
 24. The method of claim 22 wherein thesecond sub-sequence is a set of orthogonal weight vectors.
 25. Themethod according to claim 1 wherein the sequence of weight vectorsincludes weight vectors designed for transmission to the knownsubscriber units of the communication station, the designed weightvectors determined from transmit spatial signatures of the knownsubscriber units of the communication station.
 26. A method fortransmitting a downlink signal from a communication station to one ormore subscriber units, the communication station including an array ofantenna elements, each antenna element coupled to an associated transmitapparatus having an input and an output, the coupling of each antennaelement being to the output of its associated transmit apparatus, theassociated transmit apparatus inputs coupled to a signal processor, themethod comprising: for each particular signal processing procedure of aset of different signal processing procedures, each of the signalprocessing procedures being for processing the downlink signal to form aplurality of processed downlink antenna signals, each of the signalprocessing procedures including weighting the downlink signal in phaseand amplitude according to a corresponding weight vector, each processeddownlink antenna signal having an intended antenna element in the array,repeating the steps of: (a) processing the downlink signal according tothe particular signal processing procedure to form a particularplurality of processed downlink antenna signals; (b) transmitting thedownlink signal by passing each processed downlink antenna signal of theparticular plurality of processed downlink antenna signals to itsintended antenna element through the intended antenna element'sassociated transmit apparatus the set of different signal processingprocedures designed to achieve a desirable radiation level at anylocation in the complete range of azimuths of the antenna array duringat least one of the repetitions of step (b) of transmitting, eachdifferent procedure of the set of different signal processing proceduresalso comprising a set of post-processing procedures of a correspondingsequence of different sets of post-processing procedures, thecorresponding weight vectors being essentially identical for each set ofprocedures of the sequence of different sets of signal processingprocedures, and the repetition of step (a) comprising (i) weighting thedownlink signal according to the corresponding weight vector to form aplurality of downlink antenna signals, and (ii) applying a differentpost-processing procedure of one of the sets of post-processingprocedures to each of the downlink antenna signals of the plurality ofdownlink antenna signals to form each processed downlink antenna signalsof the particular plurality of processed downlink antenna signals. 27.The method of claim 26 wherein each set of post-processing procedures ofthe corresponding sequence of different sets of post-processingprocedures comprises applying a different set of phase shifts.
 28. Themethod of claim 27 wherein the phase shifts in each different set arerandom relative to each other.
 29. The method of claim 26 wherein eachset of post-processing procedures of the corresponding sequence ofdifferent sets of post-processing procedures comprises applying adifferent set of time delays.
 30. The method of claim 26 wherein eachpost-processing procedure of the corresponding set of differentpost-processing procedures comprises applying a different frequencyoffset.